1. Field of the Invention
The invention relates generally to integrated circuits having repeated logic and interconnect structures provided therein. The invention relates more specifically to providing symmetric routability to a limited number of tristateable interconnect resources within field programmable gate arrays (FPGA's).
2. Description of the Related Art
As density within integrated circuits (IC's) of digital logic circuitry increases, and as signal processing speed of such logic also increases, the ability to couple respective signals to an appropriate kinds of interconnect resource becomes more difficult.
Artisans have begun to recognize that conductors of different lengths and orientations should be provided in combination with different, line-driving amplifiers for servicing different kinds of signals in programmable logic arrays. By way of example, a first class of relatively long and relatively low-resistance conductors are included with high-powered, tristate line-drivers for broadcasting common control signals (e.g., clock, clock enable, etc.) over relatively large distances of the IC device with minimal skew. Such special conductors are sometimes referred to as tristateable, low-skew longlines.
As a further example, some wire segments are dedicated for transmitting logic input and logic output signals between immediately adjacent logic sections without routing through general switch matrices. These dedicated conductors are sometimes referred to as direct-connect lines and their correspondingly adapted line-drivers are sometimes referred to as direct-connect drivers.
At the same time that specialized conductors and line-drivers are provided, artisans strive to continue to provide field programmable logic arrays with general-purpose conductors, general-purpose routing switches and general-purpose line-drivers for carrying out general-purpose, sourcing and programmable routing of signals.
With all different kinds of conductors and line-drivers competing for space within an IC, the numbers of drivers and conductors for each kind of specialized interconnect resource (e.g., longlines) at each location can become a relatively limited resource. Every signal within a complex design cannot be allowed to have its own dedicated interconnect line and line-driver. If it were otherwise, the limited interconnect resources of the field-programmable array device would soon be exhausted. Fortunately, many designs allow for the transmission of plural signals at different times over a shared interconnect line. Such sharing may come in the form of time-domain multiplexing or burst-mode operations.
A number of different circuit techniques have been developed for allowing multiple signals to share a same interconnect line. Multiple tristate drivers may be used for example, with each tristate driver becoming a line master at a different time while the other tristate drivers of the same line go into a high-impedance output mode. The line-driving signal of that moment then passes without contention onto the shared line through its line-mastering, tristate (three state) driver.
In an alternative approach, a shared wire is urged towards a predefined logic state by means of a pull-up or pull-down resistor. An open-drain technology is then used to implement a wired-OR circuit on the urged line. Sharing signals OR into the shared line at different times. If desired, a logical ORring of simultaneous signals may be carried out on the so-driven line.
A third approach provides a dedicated multiplexer for driving the shared line. At each given time, an appropriately desired signal is selected by the dedicated multiplexer for output onto the shared line.
Each of these approaches has benefits and drawbacks. Tristate drivers tend to consume more circuit area than two-state drivers. They also generally need specialized control circuits for controlling their output-enable (OE) terminals so that contention and crowbar currents will be avoided. On the other hand, chip-internal tristate buses may be made extensible with off-chip tristate buses.
Wired-OR circuits have the drawback of tending to consume more power than purely CMOS circuits. Dedicated multiplexers are wasteful if it happens that their full selection capabilities are not utilized in a given design implementation. Also, signals on directly-multiplexed buses (without tristate capability) cannot be easily exchanged between off-chip and chip-internal circuits.
Given this, use of chip-internal tristate buses and chip-internal tristate drivers can be highly advantageous if they are used with minimal wastage of circuit space and maximal flexibility in terms of routing signals towards different directions.